Process and device for heat treatment in continuous flow of a product mixture consisting of a liquid containing solid particulates

Abstract

During a heat treatment in continuous flow of a product mixture consisting of a liquid containing solid particulates, the mixture is heated to a certain desired temperature in one or more heat exchange sections, held at this temperature in a heat-holding section for a certain time, and then cooled to the desired end temperature in one or more heat exchange sections. The residence time of the solid particulates in the heat-holding section is regulated separately and independently of the residence time of the liquid in the heat-holding section, while the solid particulates are constantly flowed round by the liquid. If there are solid particulates of various sizes in the liquid, the residence time of the various size fractions of solid particulates is regulated separately depending on the size of the solid particulates in a particular fraction. With this fraction-specific heat treatment it can be achieved that both the solid particulates of differing size and the liquid in the product mixture acquire a minimum required processing value, while at the same time it is prevented that one or more of the size fractions of solid particulates or the liquid is exposed to a high temperature for an unnecessarily long time, which can lead to deteroriation of the quality of the product mixture. The device for carrying out the process comprises a heat-holding device with one or more retaining devices. Each retaining device is designed for retaining solid particulates of a certain size and larger during a certain time and allowing the smaller particulates and the liquid to pass.

What is claimed is: 
     
       1. A process for heat treatment in continuous flow of a product mixture consisting of a liquid containing solid particulates, comprising the steps of: heating the mixture to a certain desired temperature in one or more heat exchange sections;   holding the mixture at this temperature in a heat-holding section for a certain time, the heat-holding section comprising means for controlling residence time of the solid particulates independently of liquid residence time, the residence time of the solid particulates in the heat-holding section having a longer duration than the liquid residence time and being regulated by the control means separately and independently of the residence time of the liquid in the heat-holding section, the liquid being allowed to flow substantially unimpeded through the heat-holding section without recirculation thereof while the solid particulates are constantly flowed round by the liquid; and then   cooling the mixture to the desired temperature in one or more heat exchange sections.   
     
     
       2. A process for heat treatment in continuous flow of a product mixture consisting of a liquid containing solid particulates, comprising the steps of heating the mixture to a certain desired temperature in one or more heat exchange sections, holding the mixture at this temperature in a heat-holding section for a certain time, the residence time of the solid particulates in the heat-holding section being regulated separately and independently of the residence time of the liquid in the heat-holding section, while the solid particulates are constantly flowed round by the liquid, and then cooling the mixture to the desired end temperature in one or more heat exchange sections, wherein the liquid contains solid particulates of various sizes which can be divided in a number of size fractions and the residence time of the different size fractions of solid particulates is regulated separately depending on the size of the solid particulates in a particular fraction. 
     
     
       3. The process of claim 2, wherein the residence time of a particular size fraction of solid particulates in the heat-holding section is increased as the dimensions of the solid particulates in this fraction are larger. 
     
     
       4. The process of claim 2, wherein the liquid and each of the different size fractions of solid particulates contained in the liquid are subjected to a different heat treatment. 
     
     
       5. A device for heat treatment in continuous flow of a product mixture consisting of a liquid containing solid particulates, comprising one or more heat exchangers in which the mixture of liquid and solid particulates can be heated to a certain desired temperature, a heat-holding device in which the liquid and the solid particulates can be held at this temperature for a certain time, the heat-holding device having retaining means comprising a movable retaining device having passage apertures formed therethrough to allow substantially unimpeded liquid flow therethrough without recirculation thereof, with which the residence time of solid particulates which are larger than the passage aperture in the heat-holding device can be regulated separately and independently of, and for a duration which is longer than, the residence time of the liquid in the heat-holding device, while the solid particulates are constantly flowed round by the liquid; and one or more heat exchangers in which the mixture can be cooled to the desired end temperature. 
     
     
       6. A device for heat treatment in continuous flow of a product mixture consisting of a liquid containing solid particulates, comprising one or more heat exchangers in which the mixture of liquid and solid particulates can be heated to a certain desired temperature, a heat-holding device in which the liquid and the solid particulates can be held at this temperature for a certain time, the heat-holding device having retaining means with which the residence time of solid particulates in the heat-holding device can be regulated separately and independently of the residence time of the liquid in the heat-holding device, while the solid particulates are constantly flowed round by the liquid; and one or more heat exchangers in which the mixture can be cooled to the desired end temperature; wherein the retaining means comprise several retaining devices set up in succession for retaining solid particulates, each retaining device containing retaining element provided with passage apertures, the size of the passage apertures in the retaining elements of a particular retaining device being essentially the same, while the size of the passage apertures in the retaining elements of different retaining devices is different for each retaining device. 
     
     
       7. The device of claim 6, wherein the size of the passage apertures in the retaining devices elements of the successive retaining devices decreases in the direction from the heat exchangers for heating the product mixture to the heat exchangers for cooling the product mixture. 
     
     
       8. The device of claim 6 or 7, comprising means for setting the time during which a retaining devices can retain the solid particulates separately for each retaining device. 
     
     
       9. The device of claim 6, wherein each of the retaining devices at least comprises a tank in which the product mixture stays for some time, and to which a supply line and discharge line for the product mixture are connected, and a retaining element fitted in the tank and acting selectively on a fraction of solid particulates above a certain size by means of which the speed of conveyance and consequently the residence time of these solid particulates in the tank can be regulated. 
     
     
       10. The device of claim 9, wherein the tank is essentially cylindrical and the retaining element is a retaining element which rotates at variable speed, and the axis of rotation of which coinciding with the axis of the cylindrical tank, and which is provided with at least one retaining part permeable to liquid. 
     
     
       11. The device of claim 10, wherein each retaining part has a sieve structure. 
     
     
       12. The device of claim 10, wherein each retaining part is provided with slits. 
     
     
       13. The device of claim 10, wherein each retaining part permeable to liquid is situated in a plane extending outwards from the shaft of the retaining element and parallel to said shaft and the free peripheral edge of each retaining part is situated near the inside of the cylindrical peripheral wall and the end walls of the tank. 
     
     
       14. The device of claim 13, wherein the plane in which each retaining part is situated is a flat plane. 
     
     
       15. The device of claim 13, wherein the plane in which each retaining part is situated is a curved plane. 
     
     
       16. The device of claim 13, wherein each retaining part comprises a number of pins which are spaced apart in the direction of the shaft of the retaining element and extend outwards at right angles from said shaft, the ends of said pins being situated near the inside of the cylindrical wall of the tank. 
     
     
       17. The device of claim 16, wherein the tank contains near the discharge line for the productmixture a removing part for removing the solid particulates from the retaining part, said removing part comprising a series of pins which are disposed next to each other and at a distance from each other in the lengthwise direction of the tank, and which extend from the cylindrical wall of the tank in the direction of the shaft of the retaining element and engage between the pins of the retaining part.